This invention relates generally to chair control mechanisms, and more particularly to a mechanism that synchronizes the movement of a chair seat and backrest in addition to providing independently operable height, tilt and forward travel-stop control assemblies.
It is known to provide a chair having a seat and backrest that are supported on a base independent of one another such that each can be pivoted about a different pivot axis to accommodate a person reclining in the chair. However, because the point about which the user bends is spaced from the pivot axes of the seat and backrest, sliding contact develops between the user and the seat or backrest during tilting of the chair.
The relationship between the path of travel followed by the seat and backrest during tilting is dictated in part by the placement of the pivot axes of the seat and backrest, and in part by the manner in which the seat and backrest are linked to one another. By adjusting these parameters, it is possible to synchronize the seat and backrest movement to maximize the ergonomics of the chair. In some conventional mechanisms, a condition known as bridging occurs, wherein the backrest rotates upwardly and rearwardly from the seat during tilting. This condition is uncomfortable and has a tendency to untuck the shirt or blouse of the user during tilting.
Alternatively, some chairs are subject to fall-away, wherein the backrest rotates downwardly and rearwardly from the seat, reducing lumbar support upon tilting. Yet other chairs may experience crunching, wherein the backrest rotates forwardly and into the seat, pinching the user. Still, other chairs may suffer from pushing, wherein the backrest rotates upwardly and forwardly of the rear of the seat during forward or return tilting movement, again pulling at the clothing of the user. Other prior art chairs may suffer from various combinations of bridging, fall-away, crunching, and pushing.
In chairs having the ability to tilt between upright and reclined positions, it is conventional to provide a tilt control assembly that permits the seat and backrest to be locked in a selected position. Typically, such assemblies include a lever on the mechanism housing that engages the seat or backrest support bracket of the mechanism in the locked position, blocking the support bracket from tilting in either direction. A problem associated with this type of prior art assembly occurs when the user is leaning forward in the chair, e.g. while in a tasking position such as using a keyboard at a computer, and the lever is inadvertently released. When this situation arises, the seat and backrest spring forward toward the upright position under the bias of a spring assembly typically provided on such mechanisms, potentially bumping the user from the chair. As such, "anti-launching" mechanisms are sometimes included on chair control mechanisms to reduce or eliminate such occurrences. These mechanisms must work in conjunction with the tilt control assembly to allow unlocking of the seat and backrest only when the user is seated against the backrest.